Due to environmental conservation issues, weight reduction of structural member is demanded. Among structural members, structural members for transportation apparatuses, particularly for automobile, are strongly demanded to be reduced in weight because the effects of weight reduction of structural members for automobiles are large, such as fuel consumption improvement and exhaust gas and carbon dioxide emission reduction.
Weight reduction of a structural member can be achieved by enhancing the strength of material of the structural member and thinning of the structural member. Regarding destruction of the structural member accompanying thinning of the structural member, namely, plastic deformation and fatigue failure of the structural member, the insufficient strength decreased due to thinning can be compensated by material strength enhancement of the structural member. However, regarding deflection of the structural member, namely, elastic deformation of the structural member, it is not possible to compensate insufficient rigidity decreased due to thinning by material strength enhancement of the structural member.
In particular, in the case of a structural member for automobiles, even when the material of the structural member is increased in strength to obtain the same strength of the entire structural member as that before thinning, if the geometrical structure of the entire thinned structural member is the same as that before thinning, and the rigidity of the entire thinned structural member has decreased, this causes noise and/or vibrations.
Further, among structural members for automobile, if rigidity decreases due to thinning of vehicle body member, chassis member, or the like, there is also a problem that operating stability decreases.
Therefore, to improve the strength as the entire structure without decreasing the rigidity thereof, it is necessary to enhance the strength of structural members forming the entire structure, and prevent reduction of rigidity of the entire structure by changing the geometrical structure of the entire structure.
As a method for preventing decrease in rigidity of the entire structure formed of structural members even when the structural members are thinned, it is effective to improve rigidity of coupling members, and to use a joint member having a hollow structure in particular.
As a metal processing method for obtaining a structural member having a hollow structure, there are casting, welding of plate materials, hydroforming, and so on.
Casting has a difficulty in thinning as compared to other metal processing methods. Further, elements for securing fluidity are added to an alloy for casting in either case of iron-based alloy and light-weight alloy, and thus it is difficult to enhance the strength of the material itself as compared to an elongated material or an extruded material of steel plate material or light-weight alloy. On the other hand, die-casting which allows thinning has a difficulty in obtaining a structural member having a hollow shape.
Further, when plate materials are welded to produce a structural member, although freedom in shape of the structural member is high, the welding length becomes long when the structural member is produced to have a hollow shape, and thus there is a problem that it is inferior in productivity.
Accordingly, it is effective to form the entire structure, for example the automobile body or chassis, by welding a structural joint member which is hydroformed.
Patent Document 1 and Non-patent Document 1 disclose a hydroformed body having plural projecting portions on a main tube part by shaping an element tube by hydroforming.
Further, Patent Document 2 discloses a bulge-formed product of polygonal tube for joint member, employing a hollow material formed of aluminum or an alloy thereof extruded in a polygonal shape in advance as a raw material, where two expanded portions are formed in this polygonal hollow material by bulge forming.